High-energy glass cutting

ABSTRACT

A method for severing an at least partially transparent material includes focusing ultrashort laser pulses, as individual laser pulses and/or as pulse trains, in the material so that a resulting modification zone elongated in a beam propagation direction enters the material and penetrates at least one surface of the material. Each pulse train comprises multiple sub-laser pulses, The method further includes introducing a plurality of material modifications along a severing line into the material via the laser pulses, and severing the material along the severing line, A pulse energy of the individual laser pulses or a sum of pulse energies of the sub-laser pulses is in a range from 500 μJ to 50 mJ. A length of the modification zone in the beam propagation direction is greater than a thickness of the material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/EP2021/080197 (WO 2022/122252 A1), filed on Oct. 29, 2021, andclaims benefit to German Patent Application No. DE 10 2020 132 700.5,filed on Dec. 8, 2020. The aforementioned applications are herebyincorporated by reference herein.

FIELD

Embodiments of the present invention relate to a method for severing anat least partially transparent material.

BACKGROUND

In recent years, the development of lasers having very short pulselengths, in particular having pulse lengths less than a nanosecond, andhigh average powers, in particular in the kilowatt range, has resultedin a novel type of material processing. The short pulse length and highpulse peak power or the high pulse energy of several hundred microjoulescan result in nonlinear absorption of the pulse energy in the material,so that actually transparent or essentially transparent materials canalso be processed for the laser light wavelength used.

A severing method is described in U.S. Ser. No. 10/421,683, which isbased on introducing laser pulses into the material. Methods accordingto the prior art have the problem above all that in the case of thickermaterials, in particular glasses or layered systems, having a materialthickness of greater than 1 mm, good severability is only to be achievedwith difficulty or not at all. Good severability is typically understoodto mean that a material can be reliably severed along a specifiedsevering line.

SUMMARY

Embodiments of the present invention provide a method for severing an atleast partially transparent material. The method includes focusingultrashort laser pulses, as individual laser pulses and/or as pulsetrains, in the material so that a resulting modification zone elongatedin a beam propagation direction enters the material and penetrates atleast one surface of the material. Each pulse train comprises multiplesub-laser pulses, The method further includes introducing a plurality ofmaterial modifications along a severing line into the material via thelaser pulses, and severing the material along the severing line, A pulseenergy of the individual laser pulses or a sum of pulse energies of thesub-laser pulses is in a range from 500 μJ to 50 mJ. A length of themodification zone in the beam propagation direction is greater than athickness of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

Subject matter of the present disclosure will be described in evengreater detail below based on the exemplary figures. All featuresdescribed and/or illustrated herein can be used alone or combined indifferent combinations. The features and advantages of variousembodiments will become apparent by reading the following detaileddescription with reference to the attached drawings, which illustratethe following:

FIGS. 1A, 1B, and 1C show a schematic representation of carrying out themethod according to some embodiments;

FIGS. 2A and 2B show a microscope image and cross section of a materialmodification with material ejection according to an embodiment;

FIGS. 3A, 3B, 3C, 3D, 3E, and 3F show a schematic representation of beamcross sections of quasi-non-diffracting beams according to someembodiments;

FIGS. 4A, 4B, 4C, and 4D show an analysis of the beam cross section ofquasi-non-diffracting beams according some embodiments;

FIG. 5 shows a schematic representation of a compound ellipticalquasi-non-diffracting beam according to an embodiment;

FIGS. 6A, 6B, and 6C show a further schematic representation of carryingout the method according to some embodiments;

FIGS. 7A, 7B, 7C, and 7D show schematic representations of ellipticalbeam cross sections and material modifications, and the alignmentthereof along a severing line, according to some embodiments;

FIGS. 8A and 8B show a schematic representation of the device forcarrying out the method according to some embodiments;

FIGS. 9A and 9B show a schematic representation of carrying out themethod according to some embodiments; and

FIG. 10 shows microscope pictures of material modifications generatedaccording to the method according to some embodiments.

DETAILED DESCRIPTION

Embodiments of the present invention provide a method for severing an atleast partially transparent material. Ultrashort laser pulses in theform of individual laser pulses and/or in the form of pulse trains,which comprise multiple sub-laser pulses, are focused in the material sothat the resulting modification zone, which is elongated in the beampropagation direction, enters the material and penetrates at least onesurface of the material, wherein material modifications are thusintroduced into the material, wherein a plurality of materialmodifications are introduced along a severing line into the material,and wherein the material is subsequently severed by means of a severingcut along the severing line. According to embodiments of the invention,the pulse energy of the individual laser pulses or the sum of the pulseenergies of the sub-laser pulses is in a range from 500 μJ to 50 mJ.

The material can be a metal or a semiconductor or an insulator or acombination thereof. It can also be a glass, a glass ceramic, a polymer,or a semiconductor wafer, for example a silicon wafer. The material canalso be a glass substrate and/or a stacked substrate system and/or asilicon wafer. The thickness L_(M) of the material is preferably greaterthan 1 mm.

The material is partially transparent to the wavelength of the laser inthis case, wherein partially transparent means that typically 50% ormore of the incident light of this wavelength is transmitted through thematerial.

The ultrashort pulse laser provides ultrashort laser pulses in thiscase. Ultrashort in this case can mean that the pulse length is, forexample, between 500 ps and 1 fs, or between 100 ps and 10 fs. Theultrashort pulse laser can also provide pulse trains (so-called bursts)made up of ultrashort laser pulses, wherein each pulse train comprisesthe emission of multiple sub-laser pulses. The time interval of thesub-laser pulses can in this case be between 10 ps and 500 ns, orbetween 10 ns and 80 ns. An ultrashort laser pulse is also viewed as achronologically formed pulse which has a noteworthy change of theamplitude within a range between 50 fs and 5 ps. The term pulse or laserpulse is used repeatedly in the following text. In this case,chronologically shaped laser pulses are also included, even if this isnot explicitly mentioned in each case. The ultrashort laser pulsesemitted by the ultrashort pulse laser accordingly form a laser beam.

The laser beam is focused in the material so that the laser beamincludes an elongated focus zone in the beam propagation direction. Thiscan mean that the focus zone of the laser beam in the beam propagationdirection is greater than the extension of the laser beam perpendicularto the beam propagation direction. A general definition for theextension of the focus zone is given below.

In contrast, the elongated modification zone describes the area of thelaser beam in which the intensity is above the processing threshold ofthe material, so that material processing can take place within themodification zone of the laser. The geometric shape of the modificationzone of the laser and the focus zone are linked to one another in thiscase by the scaling using the laser intensity.

The elongated modification zone can penetrate at least one surface. Thiscan mean that a surface of the material intersects the elongatedmodification zone. The intensity of the laser beam is thus greater onthis surface than on the surface which is not penetrated by theelongated modification zone. Thus, it is possible that the laser beamemits the pulse energy into the volume of the material.

The elongated modification zone can also penetrate more than onesurface. Two opposing surfaces can thus also be penetrated by theelongated modification zone, so that a quasi-homogeneous intensitydistribution by the laser exists between the two surfaces.

The laser pulse or the laser pulses are at least partially absorbed bythe material, so that the material heats up locally or enters atemporary plasma-type state. The absorption can be based in this case onlinear or nonlinear absorption. The size of the processed area isdetermined here by the beam geometry, in particular by the modificationzone of the laser beam and the beam cross section. A materialmodification can be generated by the modification zone elongated in thebeam propagation direction which can reach, for example, over the entirethickness of the material.

Such a material modification over the entire material thickness can begenerated directly, for example, using a single pulse or a single laserpulse train of sub-laser pulses. The material modifications are thusintroduced into the material by the local action of the laser.

The material modification can in this case in general be a modificationof the structure, in particular the crystalline structure and/or theamorphous structure and/or the mechanical structure, of the material.For example, an introduced material modification of an amorphousmaterial can be that the material receives a changed network structuredue to local heating only in this area. For example, the bond angles andlengths of the network structure can be changed by the modification. Amaterial modification can in particular be a local density change, whichcan also comprise areas without material, which can also be dependent onthe selected material.

In dependence on the specific material properties and the specificsettings of the laser, such as pulse energy, pulse duration, andrepetition rate, furthermore other types of material modifications canalso occur. For example, the laser can provide a laser beam at a firstsetting, which results in an isotropic index of refraction change in thematerial. The laser can also however provide a laser beam at a furthersetting, which results in a birefringent index of refraction change inthe material, so that the material has local birefringent properties.

In particular at high pulse energies, so-called micro-explosions canoccur, in the case of which highly excited, then gaseous material ispressed out of the focus zone into the surrounding material and a lessdense area or an empty core having surrounding compacted materialresults. The size of the heated area is determined here by the beamgeometry, in particular by the modification zone of the laser beam andthe beam cross section.

In contrast to the material modification, the material modification areacomprises the entire area here in which the effects of the action of thelaser pulse are measurable, for example, on the basis of the tensile andcompressive stresses. This is in particular the area in which thematerial, viewed spatially starting from the material modification,merges back into the starting state of the untreated areas of thematerial.

Due to the temperature gradients which arise due to the local pulseaction, stresses, which promote cracking, can occur in the materialmodification area upon the heating and/or upon the cooling and formationof the material modification. In particular, tensile and alsocompressive stresses can arise in the material modification area, whichextend radially or ortho-radially, for example. A material modificationis therefore preferably accompanied by indexed cracking, thus targeteddamage to the material.

As a function of the selected pulse energy, the material modificationcan generate material ejections at a surface of the material. Thematerial ejections are a measure in this case of the quality of thematerial modifications and thus also of the severability of thematerial.

Material ejections are in this case material accumulations on a surfaceof the material, which arise around the location at which the laserpulses are introduced to generate a material modification. In particular“a surface” means that it can be both the upper side and the lower sideof the material relative to the beam propagation direction here.Material ejections are a result of the heating of the material, whichpenetrates out of the volume of the material upon the introduction ofthe laser pulses. However, a part of the volume can also be lost byvaporization, etc., so that there does not have to be accuratecorrespondence of the material volumes displaced from the material andthe material volumes deposited around the material modification in thematerial ejections.

The material modifications are introduced into the material along adesired severing line. A severing line describes in this case that linealong which the material or parts of the material are to be severed orcut off.

The material is quasi-perforated by the introduced materialmodifications along a severing line in the material, so that a type ofpredetermined breaking point in the material is defined by the severingline. This perforation generally does not result in independent severingof the material, however. Rather, the material modifications along thesevering line ensure, for example, material weakening, so that uponapplication of a following severing step, for example by application ofa thermal stress and/or by application of a mechanical stress,preferably a tensile or bending stress, and/or by etching by means of atleast one wet-chemical solution, severing takes place along the severingline.

The pulse energy of the individual laser pulses or the sum of the pulseenergies of the sub-laser pulses is in a range from 500 μJ to 50 mJ. Inthis way, a good severability is achieved above all in thick materials,for example having a material thickness of greater than 1 mm.

The severing step can comprise the application of a thermal stress alongthe severing line and/or the application of a mechanical stress,preferably a tensile or bending stress, and/or etching by means of atleast one wet-chemical solution.

A thermal stress can be achieved, for example, by heating the materialalong the severing line. For example, the severing line can be heated bymeans of a continuous wave CO2 laser, so that the material in thematerial modification area expands differently in comparison to theuntreated or non-heated material. The cracks promoted by the materialmodification thus experience a crack growth, so that a continuous andnon-interlocked severing surface can form, by which the parts of thematerial are separated from one another.

A tensile or bending stress can be generated, for example, by theapplication of a mechanical load to the material parts separated by thesevering line. For example, a tensile stress can be applied when forcesopposing in the material plane act on the material parts separated bythe severing line at a force engagement point in each case, each ofwhich points away from the severing line. The forces are thus notaligned in parallel or antiparallel to one another, so that this cancontribute to the occurrence of a bending stress. As soon as the tensileor bending stresses are greater than the bonding forces of the materialalong the severing line, the material is severed along the severingline.

The material can also be severed by etching using a wet-chemicalsolution, wherein the etching process preferably attacks the material atthe material modification, thus the targeted material weakening. In thatthe material parts weakened by the material modification are preferablyetched, this results in severing of the material along the severingline.

This has the advantage that an ideal severing method can be selected forthe respective material, so that severing of the material is accompaniedby a high quality of the severed edge.

The laser pulses can have a wavelength between 0.3 μm and 1.5 μm, and/orthe pulse length of the individual laser pulses and/or the sub-laserpulses can be 0.01 ps to 50 ps, preferably 0.3-15 ps, and/or the averagepower of the laser at the laser output can be between 150 W and 15 kW.

This has the advantage that the method can be optimized for therespective material over a large parameter range. In particular, thisincreases the probability of finding a laser wavelength available for amaterial, at which the material is partially transparent.

The laser beam formed by the laser pulses and the material can bedisplaceable relative to one another with a feed in order to introducethe plurality of the material modifications along the severing line intothe material, wherein the laser beam and the material are preferablyalignable in relation to one another at an angle, in particular tiltableand/or rotatable.

Displaceable relative to one another means both that the laser beam canbe translationally displaced relative to a stationary material, and thematerial can also be displaced relative to the laser beam, or a movementtakes place of both the material and the laser beam.

In particular, the focus of the laser beam can thus be placed at variouslocations of the material in order to introduce material modifications.In addition to translational movements along the X, Y, and Z axes,rotational movements are also possible in particular, in particularrotations of the material around the beam propagation direction. Thiscan comprise rotations around all Euler angles.

It is thus possible to orient the laser beam along the severing line.

In one preferred embodiment, the elongated modification zone is longerin the beam propagation direction than the material thickness L_(M), inparticular longer than 1.5×L_(M) or longer than (2×200 μm)+L_(M).

In that the elongated modification zone is longer than the materialthickness, the material modification can be introduced over the entirematerial thickness. In particular, a large focus location tolerance canalso be achieved, so that material thickness variations or materialirregularities, in particular in large-format glass substrates having asize of greater than 1 m², can be neglected. However, it is to be notedthat the required pulse energy for introducing a material modificationrises linearly with the length of the focus zone.

The maximum diameter of the beam cross section perpendicular to the beampropagation direction in the modification zone can be between 1 μm and50 μm, preferably between 2 μm and 4 μm.

In particular material modifications having a large lateral extensioncan thus be generated, so that the severability of the material isimproved.

The laser beam formed by the laser pulses can, at least in the elongatedfocus zone, be a quasi-non-diffracting beam or a coherent superpositionof at least two quasi-non-diffracting beams.

Non-diffracting beams satisfy the Helmholtz equation:

∇² U(x,y,z)+k ² U(X,y,Z)=0

and have a clear separability into a transverse and a longitudinaldependence of the form

U(x,y,z)=U _(t)(x,y)exp(ik _(z) z).

In this case, k=ω/c is the wave vector having its transverse andlongitudinal components k²=k_(z) ²+k_(t) ² and U_(t)(x,y) is anarbitrary complex-valued function, which is only dependent on thetransverse coordinates x,y. The z dependence in the beam propagationdirection in U(x,y,z) results solely in a phase modulation, so that theassociated intensity I of the solution is propagation invariant ornon-diffracting:

I(x,y,z)=|U(x,y,z)|² =I(x,y).

This approach provides various solution classes in different coordinatesystems, such as Mathieu beams in elliptical-cylindrical coordinates orBessel beams in circular-cylindrical coordinates.

A plurality of non-diffracting beams may be experimentally implementedin good approximation, thus quasi-non-diffracting beams. In contrast tothe theoretical construct, these only conduct a finite power. The lengthL of the propagation invariance of these quasi-non-diffracting beams isalso finite.

Based on the norm for laser beam characterization ISO11146 1-3, the beamdiameter is determined via the so-called second moments. The power ofthe laser beam or also the zero-order moment is defined in this case as:

P=∫dx dy I(x,y).

The first-order spatial moments indicate the focal point of theintensity distribution and are defined as:

${\langle x \rangle = {\frac{1}{P}{\int{{dxdyxI}( {x,y} )}}}},$$\langle y \rangle = {\frac{1}{P}{\int{{{dxdyyI}( {x,y} )}.}}}$

Based on the above equations, the second-order spatial moments of thetransverse intensity distribution may be calculated:

${\langle x^{2} \rangle = {\frac{1}{P}{\int{{{dxdy}( {x - \langle x \rangle} )}^{2}{I( {x,y} )}}}}},$${\langle y^{2} \rangle = {\frac{1}{P}{\int{{{dxdy}( {y - \langle y \rangle} )}^{2}{I( {x,y} )}}}}},$$\langle {xy} \rangle = {\frac{1}{P}{\int{{{dxdy}( {x - \langle x \rangle} )}( {y - \langle y \rangle} ){{I( {x,y} )}.}}}}$

The beam diameter or the size of the focus zone in the main axes may bedetermined using the second-order spatial moments of the laser beam thuscompletely defined. The main axes are in this case the directions of theminimum and maximum extension of the transverse beam profile, thus theintensity distribution perpendicular to the beam propagation direction,which always extend orthogonally to one another. The focus zone d of thelaser beam then results as follows:

${d_{x} = {2\sqrt{2}\{ {( {\langle x^{2} \rangle + \langle y^{2} \rangle} ) + {\gamma\lbrack {( {\langle x^{2} \rangle - \langle y^{2} \rangle} )^{2} + {4( \langle {xy} \rangle )^{2}}} \rbrack}^{\frac{1}{2}}} \}^{\frac{1}{2}}}},$${d_{y} = {2\sqrt{2}\{ {( {\langle x^{2} \rangle + \langle y^{2} \rangle} ) - {\gamma\lbrack {( {\langle x^{2} \rangle - \langle y^{2} \rangle} )^{2} + {4( \langle {xy} \rangle )^{2}}} \rbrack}^{\frac{1}{2}}} \}^{\frac{1}{2}}}},$with$\gamma = {\frac{\langle x^{2} \rangle - \langle y^{2} \rangle}{❘{\langle x^{2} \rangle - \langle y^{2} \rangle}❘}.}$

In particular, a long and a short main axis of the transverse focus zoneresult by way of the values d_(x) and d_(y).

The focus zone of a Gaussian beam is thus defined via the second momentsof the beam. In particular, the size of the transverse focus zone d^(GF)_(x,y) and the longitudinal extension of the focus zone, the Rayleighlength z_(R), result therefrom. The Rayleigh length z_(R) is given byz_(R)=π(d^(GF) _(x,y))²/4λ. It describes the distance along the beampropagation direction starting from the position of the intensitymaximum, at which the area of the focus zone has increased by the factorof 2. In the case of a symmetrical Gaussian beam, the following appliesfor the focus zone: d^(GF) ₀=d^(GF) _(x)=d^(GF) _(y).

Furthermore, we define as the transverse focus diameter inquasi-non-diffracting beams d^(ND) ₀ the transverse dimensions of localintensity maxima as twice the shortest distance between an intensitymaximum and an intensity drop to 25% starting therefrom.

The focus zone of the quasi-non-diffracting beams is also defined viathe second moments of the beam. In particular, the focus zone resultsfrom the size of the transverse focus zone d^(ND) _(x,y) and thelongitudinal extension of the focus zone, the so-called characteristiclength L. The characteristic length L of the quasi-non-diffracting beamis defined via the intensity drop to 50%, starting from the localintensity maximum, along the beam propagation direction. In particular,the size of the focus zone is normed as shown above to the total laserpower and is thus independent of the maximum power which is transportedby the beam.

A quasi-non-diffracting beam exists precisely when d^(ND) _(x,y)≈d^(GF)_(x,y), thus similar transverse dimensions which significantly exceedthe characteristic length L of the Rayleigh length of the associatedGaussian focus, for example, if L>10z_(R).

Quasi-Bessel beams or Bessel-like beams, also called Bessel beams here,are known as a subset of the quasi-non-diffracting beams. In this case,the transverse field distribution U_(t)(x,y) in the vicinity of theoptical axis obeys in good approximation a Bessel function of the firsttype of the nth order. A further subset of this class of beams isrepresented by the Bessel-Gauss beams, which are widespread due to theirsimple generation. The illumination of an axicon in refractive,diffractive, or reflective embodiment using a collimated Gaussian beamthus permits the formation of the Bessel-Gauss beam. The associatedtransverse field distribution in the vicinity of the optical axis obeysin good approximation a zero-order Bessel function of the first type,which is enclosed by a Gaussian distribution.

Accordingly, it can be advantageous to use a quasi-non-diffracting beam,in particular a Bessel beam, to process a material, since a large focuslocation tolerance can be achieved in this way.

Typical Bessel-Gauss beams for processing a material have, for example,a d^(ND) _(x,y)=2.5 large transverse focus zone, whereas thecharacteristic length can be 50 For a Gaussian beam having a d^(GF)_(x,y)=2.5 μm large transverse focus zone, the Rayleigh length in aircan only be z_(R)≈μm at λ=1 μm, however. In these cases relevant formaterial processing, accordingly L>>10z_(R) can apply.

A coherent superposition of the quasi-non-diffracting radiation resultsin particular by superposition of at least two quasi-non-diffractingbeams. It is thus possible to generate further beam profiles and thusforms of the material modifications.

The laser beam can have a non-radially symmetric beam cross sectionperpendicular to the beam propagation direction, wherein the beam crosssection or the envelope of the beam cross section is preferablyelliptical in shape.

Non-radially symmetric in this case means, for example, that thetransverse focus zone is stretched in one direction. A non-radiallysymmetric focus zone can also mean, however, that the focus zone is, forexample, cross-shaped or is triangular or N-polygonal, for examplepentagonal. A non-radially symmetric focus zone can moreover comprisefurther rotationally symmetric and mirror-symmetric beam cross sections.

For example, an elliptical focus zone can exist perpendicularly to thepropagation direction, wherein the ellipse has a long axis d_(x) and ashort axis d_(y). An elliptical focus zone thus exists when the ratiod_(x)/d_(y) is greater than 1, in particular is d_(x)/d_(y)=1.5. Theelliptical focus zone of the specific existing beam can correspond to anideal mathematical ellipse. The present specific focus zone of thequasi-non-diffracting beam can also only have the above-mentioned ratiosof long main axis and short main axis, however, but a differentcontour—for example an approximated mathematical ellipse, a dumbbellshape, or another symmetrical or asymmetrical contour, which is enclosedby a mathematically ideal ellipse.

In particular, elliptical quasi-non-diffracting beams may be generatedvia quasi-non-diffracting beams. Elliptical quasi-non-diffracting beamshave special properties in this case, which result from the analysis ofthe beam intensity. For example, elliptical quasi-non-diffracting beamshave a main maximum which coincides with the center of the beam. Thecenter of the beam is given in this case by the location at which themain axes intersect. In particular, elliptical quasi-non-diffractingbeams can result from the superposition of multiple intensity maxima,wherein in this case only the envelope of the participating intensitymaxima is elliptical. In particular, the individual intensity maxima donot have to have an elliptical intensity profile.

The secondary maxima closest to the main maximum, which result from thesolution of the Helmholtz equation, have in this case a relativeintensity of greater than 17%. Therefore—depending on the transportedlaser energy in the main maximum, enough laser energy is also conductedin the secondary maxima that material processing is enabled. Moreover,the closest secondary maxima always lie on a straight line which isperpendicular to the long main axis, or is parallel to the short mainaxis, and extends through the main maximum.

In particular, the contours of the beam cross sections have locationshaving different curve radii. For example, in an elliptical beam crosssection, the curve radius at the point at which the small half-axisintersects the ellipse is particularly large, while the curve radius atthe point at which the large half-axis intersects the ellipse isparticularly small. For example, the possibility can result for materialstresses to relax at the points of small curve radii, for example peaksand corners, so that induced cracking occurs there. It is possible toimprove the severability of the material along the severing line by acontrolled crack propagation between the material modifications.

The long axis of the non-radially symmetric beam cross section can beoriented perpendicular to the beam propagation direction along thesevering line and/or along the feed direction.

Cracking typically takes place along a preferred direction of thenon-radially symmetric beam cross section—for example, crack propagationprimarily takes place in the direction of a longer extension of the beamcross section, which is accompanied by smaller radii of the contour ofthe beam cross section at the outer contour edges located in thispreferred direction.

In particular, targeted crack guidance can be promoted by a rotation ofthe non-radially symmetric beam cross section and/or the material, sothat a preferred direction of the non-radially symmetric beam crosssection is always oriented along the severing line due to the rotation.

If the feed direction between laser beam and material is aligned, forexample, perpendicular to an axis along which a preferred crackpropagation takes place, meeting of the cracks of adjacent materialmodifications is then improbable. If the feed direction is aligned inparallel to the axis of the preferred crack propagation, in contrast, itis then probable that the cracks of adjacent material modifications willmeet and unite. Targeted crack guidance over the entire length of thesevering line can thus also be ensured by the rotation of the beam crosssection and/or the workpiece with curved severing lines. It is thuspossible to sever the material along arbitrarily shaped severing lines.

The long axis of the non-radially symmetric beam cross section can havea negligible or non-negligible intensity and can preferably have aninterference contrast of less than 0.9 in the case of the non-negligibleintensity.

An elliptical quasi-non-diffracting beam can have a non-negligibleintensity along the long main axis in this case, in particular can havean interference contrast I_(max)−I_(min)/(I_(max)+I_(min))<0.9, so thatthe beam transports laser energy everywhere along the long main axis.

I_(max) is in this case the maximum beam intensity along the long mainaxis, while I_(min) is the minimum beam intensity. If I_(min)=0, thencomplete interference occurs along the long main axis and aninterference contrast of 1 results. If I_(min)>0, then only partial orno interference occurs along the long main axis, so that theinterference contrast is <1.

If, for example, the interference contrast along the long main axis isless than 0.9, complete interference does not occur along the long mainaxis, but only partial interference, which does not result in completecancellation of the laser intensity at the location of the intensityminimum I_(min). This is the case, for example, if thequasi-non-diffracting beam is generated using a birefringent element,for example a quartz angle displacer or a quartz beam displacer or acombination thereof.

An elliptical quasi-non-diffracting beam can also have a negligibleintensity and an interference contrast of 1 along the long main axis,however, so that the beam does not transport laser energy everywherealong the long main axis. This is the case, for example, if thequasi-non-diffracting beam is generated using a modified axicon.

The laser beam formed by the laser pulses can be incident on thematerial surface at a processing angle which is preferably not a rightangle, wherein the processing angle is less than 20° for materialthicknesses less than 2 mm and is less than 10°, in particular less than5°, for material thicknesses greater than 2 mm.

In that the laser beam is incident at an angle on the material surface,the laser beam experiences a refraction upon entering the material.Accordingly, the material modification is not introduced perpendicularlyto the surface, but rather at a refraction angle which is determinedaccording to Snell's law of refraction. In this way, it is possible forthe material not to have edges which are shaped at a right angle. Forexample, beveled edges can be generated along which materials can beassembled again and joined, for example. For example, lateral joining ofmaterials to one another can thus be achieved.

In particular, the processing angle of the modification zone in thematerial, in which good severability is still achieved, is dependent onthe material thickness.

The individual laser pulses and/or pulse trains can be triggered by aposition-controlled pulse triggering of the laser system, wherein theposition is preferably provided by the position of the laser beam formedby the laser pulses on the material.

A position-controlled pulse triggering can be implemented via adetector, which reads the location of the material or the feed device orthe feed vector and the position of the laser beam.

It is thus possible for material modifications to be introduced into thematerial along the severing line at equal intervals. It is thus possiblein particular to prevent material modifications from overlapping, as canoccur with a constant laser pulse rate and varying feed speed.

Preferred exemplary embodiments are described hereinafter on the basisof the figures. Identical, similar, or identically acting elements areprovided with identical reference signs in the different figures and arepeated description of these elements is partially omitted to avoidredundancies.

FIG. 1 schematically shows the severing method described here forsevering an at least partially transparent material 1.

To sever the material 1, laser pulses of an ultrashort pulse laser 6(see FIG. 8A, for example) are focused in the material 1. The laserpulses run in the laser beam 60, which are absorbed at least partiallyby the material 1 in the modification zone 602 of the laser beam 60, inorder to introduce a material modification 3 into the material 1 in thisway. The shaded plane in this case shows the plane below the severingline 2, along which the material 1 is severed. Ideally, this planecorresponds to the later severing surface 20.

Material modifications 3 can be generated due to the linear and/ornonlinear absorption of the laser pulses in the material 1. For example,the general structure of the material 1 or the density of the materialcan thus be changed in order to form the material modifications 3 inthis way.

However, it is also possible that so-called micro-explosions occur dueto the absorption of the laser pulses, in which the material 1 issuddenly vaporized in the modification zone 602 of the laser beam. Thehighly excited, then gaseous material 1 is moved into the surroundingmaterial 1 by the high pressure, so that the material 1 is compacted atthe shock front. A less dense or empty core (“void”), which issurrounded by the compacted material, thus arises in the area of themodification zone 602. In particular, a part of the material can alsopenetrate outward from the modification zone 602 due to themicro-explosions, where it is deposited on the surface of the material 1and forms material ejections 300.

These modifications result in the material modification 3. A materialmodification area 30 is formed around the material modification 3. Inthe material modification area 30, the material gradually passes fromthe state which is present in the material modification 3 back into itsoriginal state, the farther away the material is observed from thematerial modification 3. The original state can be, for example, theunprocessed state of the material, which is present in adjacent pointsin the material 1, for example. The original state is also understoodhere, however, as the state of the material 1 which was present beforethe introduction of the material modification 3.

The laser pulses can have a wavelength between 0.3 μm and 1.5 μm and/orthe pulse length of the laser pulses can be 0.01 ps to 50 ps, preferablycan be 0.3-15 ps, and/or the average power of the laser can be 150 W to15 kW. The laser energy can be introduced in the form of individuallaser pulses into the material, wherein the repetition rate of theindividual laser pulses is 1 kHz to 2 MHz. However, the laser energy canalso be introduced into the material in the form of pulse trains,comprising multiple sub-laser pulses, wherein the repetition frequencyof the sub-laser pulses of the pulse train can be between 2 MHz and 100GHz, in particular 12.5 MHz to 100 MHz, furthermore wherein a pulsetrain can preferably comprise 2 to 20 sub-laser pulses and/or the sum ofthe pulse energies of the sub-laser pulses of a pulse train can bebetween 500 μJ and 50 mJ.

For example, a material modification 3 can be generated using a laserhaving 1 μm wavelength, a pulse duration of 1 ps, and an average powerof 1000 W. The laser pulse can be introduced in the form of anindividual pulse into the material 1, wherein the repetition rate of thelaser is, for example, 100 kHz.

Local stresses can occur in the material modification 3 and the materialmodification area 30, which promote cracking. For example, the material1 can have a different density—for example a lower density—due to localheating and can thus build up a compressive stress in the materialmodification area 30. However, a higher density can also exist in theheated area and a tensile stress can thus be built up in the materialmodification area 30. If the tensile and/or compressive stress becomesexcessively large, for example greater than the tensile or compressivestrength of the untreated material, a crack can form spontaneously.

As shown in FIG. 1 , multiple material modifications 3 are introducedinto the material 1. Material modification areas 30 form around eachmaterial modification 3. The placement of the material modifications 3takes place in this case along the desired severing line 2. The severingline 2 is an imaginary line along which the material 1 is to be severed.

The material 1 is quasi-perforated by the introduced materialmodifications 3 along the severing line 2 in the material 1, so that atype of predetermined breaking point in the material 1 is defined by thesevering line 2. This perforation generally does not result inindependent severing of the material 1, however. Rather, the materialmodifications 3 along the severing line 2 ensure, for example, targetedmaterial weakening and/or a targeted introduction of cracks 32, whichinduce material weakening along the severing line 2.

After the material modifications 3 are introduced into the material 1 bymeans of the laser beam 6, for example, in a following severing step,the material 1 can be physically severed by applying a tensile force FZto the material halves 10 and 12 separated from one another by thesevering line 2. In particular, it is also possible to sever thematerial 1 by applying a bending stress to the material halves 10, 12(not shown).

FIG. 1B shows an analogous method, in which the material halves are notsevered using a mechanical force in a severing step, but rather byapplying a thermal stress.

After the material modifications 3 have been introduced, a thermalgradient 620 can be generated via the material modifications 3. Acontinuous wave CO2 laser 62, for example, can be used to introduce thethermal gradient 620.

The focus of the continuous wave CO2 laser 62 can be placed, forexample, a few micrometers below the surface 14 to generate the thermalgradient 620, so that the severing of the material 1 runs with littledamage and a smooth fracture edge or severing surface 20 results.However, the focus can also be positioned at a different distance to thesurface. In general, a large part of the continuous wave CO2 laserradiation is already absorbed a few nanometers below the surface of thematerial, so that there is at least no strong dependence on thepositioning of the focus of the continuous wave CO2 laser 62.

Due to the dominant absorption in the vicinity of the upper surface 14of the material, the temperature is greater there than at the lowersurface. A thermal gradient T(z) thus results. Due to the thermalexpansion of the material 1, which is linearly dependent on thetemperature in a first approximation, the material 1 expands morestrongly at the upper surface 14 than at the lower surface. Materialstresses of different strengths thus occur along the Z axis.

The various material stresses run through the introduced materialmodifications 3. Material stresses can preferably relax there, whichresults in cracking. The cracking takes place between the variousadjacent material modifications 3. Cracking thus occurs which ultimatelysevers the material 1 into the two material halves 10 and 12.

FIG. 1C shows a further analogous method, in which the material halves10, 12 are severed in a severing step by means of a wet-chemicalreaction. For this purpose, the material 1 perforated using the materialmodifications 3 is put into a chemical bath 11. The chemical bath 11contains in this case a solvent which is capable of removing and etchingthe material 1. In particular, the etching procedure takes place in thepreviously introduced material modifications 3, since the materialweakening is particularly large there and the change of the physicaland/or chemical properties at the location of the material modification3 causes the reaction to run particularly advantageously. A materialmodification 3 can in a certain sense act as a catalyst of the etchingreaction. The reaction is schematically indicated in FIG. 1C by theoccurrence of reaction bubbles 110 in the chemical bath 11.

As soon as the material 1 is etched through, the material 1 is severedinto two material halves 10, 12. If the material 1 is not yet severedafter the chemical bath 11, for example since the chemical bath 11 hasexclusively etched away the material modifications 3, the material 1 hasthus been deliberately damaged further along the severing line 2, sothat the material 1 can be severed into the material halves 10, 12 byapplying a tensile or bending stress, for example.

FIG. 2A shows a microscope image of the surface of a processed material1. Round material modifications 3 were introduced into the material 1along the severing line 2 at an interval dM=5 μm. The materialmodifications 3 have the shape of a perforated channel, wherein thematerial of the outer lateral surface of the perforated channel wascompacted by micro-explosions during the introduction of the materialmodification 3. Round material ejections 300 result on the surface ofthe material 1 around the round opening of the material modification 3or the perforated channel. These material ejections 300 have an externaldiameter dA. The external diameter of the material ejections 300 is 3 μmhere.

FIG. 2B shows a thickness cross section through FIG. 2A. It can be seenclearly that the material ejections have a height above the surface ofthe material 1 of 50 nm to 200 nm. The diameter and the height of thematerial ejections 300 are specified in this case by the pulse energyand the beam cross section of the laser beam. It is apparent inparticular that the material modification 3 begins at the upper surface14. This is a result of the elongated modification zone 602 penetratingthe surface 14, thus in particular that there is a common intersectionsurface.

FIG. 3A shows the intensity curve and beam cross section 4 of aquasi-non-diffracting laser beam. In particular, thequasi-non-diffracting laser beam is a Bessel-Gauss beam. TheBessel-Gauss beam has a radial symmetry in the beam cross section 4 inthe x-y plane, so that the intensity of the laser beam is only dependenton the distance to the optical axis. In particular, the transverse beamdiameter d^(ND) _(x,y) is between 0.25 μm and 10 μm in size.

FIG. 3B shows the longitudinal beam cross section 4, thus the beam crosssection 4 in the beam propagation direction. The beam cross section 4has an elongated focus zone, which is approximately 3 mm in size. Thefocus zone is thus significantly larger in the propagation directionthan the beam cross section 4, so that an elongated focus zone 600 ispresent.

FIG. 3C shows, similarly to FIG. 3A, a non-diffracting beam, which has anon-radially symmetric beam cross section 4. In particular, the beamcross section 4 appears stretched, nearly elliptical, in the ydirection.

FIG. 3D shows the longitudinal focus zone 600 of the Bessel beam, whichagain has an extension of approximately 3 The Bessel beam alsoaccordingly has an elongated focus zone in the beam propagationdirection.

FIG. 3E shows a coherent superposition of various quasi-non-diffractingbeams. Beam profiles, which could not be achieved using a single laserbeam, can be generated by the superposition of multiplequasi-non-diffracting beams. The designations of the intensity maxima inthe x-y plane indicate the rounded intensity distribution relative tothe total intensity.

FIG. 3F shows the intensity curves of two laser beams having differentlaser power but having identical Gauss-Bessel-shaped beam cross sectionin the z direction. Both beam profiles have the same characteristiclength L, since this is defined via the drop of the laser intensity to50% of the intensity maximum. However, the material itself has aspecific intensity threshold IS, from which processing of the materialcan take place. The length of the modification zone 602 is defined inthis case as the length over which the intensity of the laser beam isabove the intensity threshold IS of the material. A large modificationzone 602 of the laser beam thus results for high laser powers, while thelaser beam has a small modification zone 602 for low laser powers. Themodification zone 602 of the laser beam thus scales with the transportedlaser power.

FIG. 4 shows a detailed analysis of the beam cross section 4 from FIG.3C, D. FIG. 4A shows the transverse intensity distribution of the laserbeam 60, wherein the main maximum and the secondary maxima result fromthe solution of the Helmholtz equation.

FIG. 4B shows the so-called iso-intensity lines of the intensitydistribution from FIG. 4A, wherein the lines are drawn where therelative intensity of the laser beam is 25% or 50% or 75%. It is clearlyvisible that the main maximum 41 of the intensity distribution has anapproximately elliptical shape, wherein the extension along the x axisis significantly greater than the extension along the y axis. Inparticular, the main maximum is adjoined by two kidney-shaped secondarymaxima 43, which have a significantly lower relative intensity.

FIG. 4C shows a cross section through the intensity distribution fromFIG. 4A through the center of the main maximum along the x axis. In thecenter of the main maximum 41, the intensity distribution has itsmaximum, wherein the relative intensity is at 100% here by definition.The intensity distribution drops along the positive and negative xdirection until at approximately 0.003 mm, a minimum in the relativeintensity distribution is reached, which is different from 0%, however.Accordingly, laser energy is also transported between the main maximum41 and the secondary maxima 43 of the laser beam 60.

FIG. 4D shows a cross section through the intensity distribution fromFIG. 4A through the center of the main maximum 41 along the y axis.However, the intensity maximum is again to be found here in the center,but the intensity drop along the y direction is significantly faster, sothat the intensity minimum is reached at approximately 0.002 mm. Theintensity minimum is exactly zero in this case, since completeinterference exists for the laser beam 60 here. In particular, secondarymaxima 43 are again to be found at larger values on the y axis, whichare above a relative intensity value of 25%, for example. This is notthe case in the x axis cross section from FIG. 4C. The properties of theelliptical beam cross section 4 therefore differ along the variouspropagation directions.

In particular, FIGS. 4C and 4B show that the long half-axis a ismeasured from the center of the main maximum to the drop of the relativeintensity to 50%. Similarly, the length of the short half-axis b ismeasured from the center of the main maximum to the drop of the relativeintensity to 50%. The long and short half-axes are perpendicular to oneanother in this case.

FIG. 5 shows that elliptical quasi-non-diffracting beams can result fromthe superposition of multiple intensity maxima, wherein in this caseonly the envelope of the participating intensity maxima is elliptical.In particular, the individual intensity maxima do not have to have anelliptical intensity profile.

In the present case, the beam cross section also has two kidney-shapedsecondary maxima 43 in addition to the pronounced main maximum 41. Up to17% of the laser energy of the main maximum 41 is transported in thesecondary maxima. If the laser pulse energy is large enough, the laserpulse energy transported in the secondary maxima 43 is also sufficientto induce a material modification 3. The geometrical shape of themodification zone 602 can thus be influenced with the selection of thelaser pulse energy.

For example, the laser pulse energy can be selected so that the areasabove the 25% iso-intensity lines can already introduce materialmodifications. The main maximum 41 and the two secondary maxima 43 theneach form, for example, overlapping material modification areas 30, sothat overall an elliptical material modification 3 results, the longaxis of which extends in the y direction. Cracking is thus also to beexpected along the y direction.

In particular, an elliptical material modification 3 will also resultdue to this, the long axis of which is analogously aligned along the yaxis.

FIGS. 6A, B show that the elongated modification zone 602 can beintroduced in different ways into the material 1. In FIG. 6A, theelongated modification zone 602 has a greater length than the materialis thick. In particular, the elongated modification zone 602 is greaterthan 1.5×L_(M). It is thus possible to position the modification zone602 so that the modification zone 602 penetrates the upper surface 14and the lower surface. It is thus possible in particular that thematerial modification 3 is introduced over the entire material thicknessL_(M). This results in a lower required severing force in the subsequentsevering process and thus a lower surface roughness of the severingsurface 20.

FIG. 6B shows that the material 1 can be constructed from various layers1′, 1″, 1′″. Each layer has a separate material thickness in this case,wherein the total material thickness L_(M) is the sum of the thicknessesof the individual layers. In particular, each layer can also have anindividual index of refraction, wherein each layer is partiallytransparent to the wavelength of the laser, however. The elongatedmodification zone 602 is also greater than the total material thicknesshere.

FIG. 6C shows that the elongated modification zone 602 can also beintroduced into the material 1 so that only one material surface 14 ispenetrated by the elongated modification zone 602. In the present case,the upper surface 14 is penetrated. However, it is also possible thatother types of material modifications 3 are introduced into the material1 by the laser beam 6.

FIG. 7A shows an elliptical material modification 3 in a material 1. Thematerial modification 3 is introduced by the laser beam 60 of the laser6 into the material 1. The shape of the material modification 3 isspecified in this case by the beam cross section 4 of the laser beam 60,in particular by its modification zone 602. Around the area of thematerial modification 3, in which a direct action of the laser beam 60exists on the material 1 for the time of the laser pulse, a materialmodification area 30 is formed, which corresponds to the shape of theintroduced material modification 3, or the beam cross section 4 of thelaser beam 6.

Accordingly, material stresses can occur both in the materialmodification 3 itself and in the material modification area 30, whichpromote cracking. For example, with an elliptical material modification3, cracking can be promoted at the points of the ellipse at which thecurve radius of the boundary line is particularly small. It is ensuredby a small curve radius that the stress which is introduced into theglass 1 by the material modification 3 can drop particularly quickly inmany different directions. A relaxation of the material stress thustakes place with higher probability at this point than at locationswhere the material stress can relax in only a few directions. The pointsof the material modification 3 which have a small curve radius are thusparticularly unstable in the material 1.

The formation of the crack 32 then preferably takes place in thedirection of the long axis of the elliptical material modification 3. Itis thus possible to control the crack propagation by way of theorientation of the material modification 3. It is thus possible inparticular to control the crack propagation from one materialmodification 3 to another material modification 3.

In FIG. 7B, multiple material modifications 3 have been introduced intothe material 1. The material modifications 3 are once again elliptical.The cracks 32 thus preferably form along the long axis of the ellipse atthe points of the smallest curve radii of the ellipse. The materialmodifications 3 are placed so close to one another in the figure thatthe respective cracks of adjacent material modifications overlap. It isthus possible that the cracks merge and form a common crack between twoadjacent material modifications. In particular, this state can beachieved by a crack growth, for example, by applying a tensile force.For example, cracks 32 can be introduced along arbitrary severing lines2 in the material 1 by this method.

FIG. 7C shows that the long axes of the material modifications 3 and thematerial ejections 300 are aligned along the severing line 2. Since thelong axes of the material modifications 3 are aligned along the severingline 2, this means at the same time that during the introduction of thematerial modifications 3, the long axis of the beam cross section of thelaser beam 60 was aligned along the severing line 2.

FIG. 7D accordingly shows that the long axis of the beam cross section 4is aligned in parallel to the feed speed V, so that the long axis isalways aligned in parallel to the severing line 2.

FIG. 8A shows a structure for carrying out the method. The laser beam 60of the ultrashort pulse laser 6 is deflected by a beamforming opticalunit 9 and an optional mirror 70 onto the material 1. The material 1 isarranged in this case on a support surface of the feed device, whereinthe support surface preferably neither reflects nor absorbs nor stronglyscatters back into the material 1 the laser energy which the materialdoes not absorb.

In particular, the laser beam 60 can be coupled by a free space sectionhaving a lens and mirror system into the beamforming optical unit 9. Thelaser can also however be coupled by a hollow core fibre 65 havingcoupling and decoupling optical units into the beamforming optical unit,as shown in FIG. 8B.

The beamforming optical unit 9 can be, for example, a diffractiveoptical element or an axicon, which generates a non-diffracting laserbeam 60 from a Gaussian laser beam 60. In the present example, the laserbeam 60 is deflected by the mirror 70 in the direction of the material 1and focused by a focusing optical unit 72 on or in the material 1. Thelaser beam 60 causes material modifications 3 in the material 1. Thebeamforming optical unit 9 can be rotated in particular, so that, forexample, a preferred direction or an axis of symmetry of the laser beamcan be adapted to the feed trajectory.

The feed device 8 can move the material 1 below the laser beam 60 with afeed V in this case, so that the laser beam 60 introduces materialmodifications 3 along the desired severing line 2. In particular, in thefigure shown, the feed device 8 comprises a first part 80 which can movethe material 1 along an axis. In particular, the feed device can alsohave a second part 82, which is configured to rotate the laser beam 60around the z axis, or around the beam propagation direction, so that thelong axis of the beam cross section perpendicular to the beampropagation direction is always tangential to the desired severing line2, in order to thus cause crack propagation along the severing line 2.

Insofar as the orientation of the long axis of the beam cross sectioncan be determined both by the beamforming optical unit 9 and by thesecond part 82 of the feed device, it is thus also possible to useeither the orientation possibility of the beamforming optical unit 9 orof the second part 82 of the feed device. However, both possibilitiescan also be used in complement to one another.

For this purpose, the feed device 8 can be connected to a control device5, wherein the control device 5 converts the user commands of a user ofthe device into control commands for the feed device 8. In particular,predefined cutting patterns can be stored in a memory of the controldevice 5 and the processes can be automatically controlled by thecontrol device 5.

The control device 5 can in particular also be connected to the laser 6.The control device 5 can in this case set the laser pulse energy of thelaser pulses of the laser 6 or request or trigger the output of a laserpulse or laser pulse train. The control device 5 can also be connectedto all mentioned components and thus coordinate the material processing.

In particular, a position-controlled pulse triggering can thus beimplemented, wherein an axis encoder of the feed device 8 is read outand the axis encoder signal can be interpreted by the control device asa location specification, for example. It is thus possible that thecontrol device 5 automatically triggers the emission of a laser pulse orlaser pulse train when, for example, an internal adding unit, which addsthe covered distance, reaches a value and resets to 0 after reaching it.Thus, for example, a laser pulse or laser pulse train can be emittedinto the material 1 automatically at regular intervals.

In that the feed speed and the feed direction and thus the severing line2 are also processed in the control device 5, the laser pulses or laserpulse trains can be emitted automatically.

The control device 5 can also calculate a distance dM or location, atwhich a laser pulse train or laser pulse is to be emitted, on the basisof the measured speed and the base frequency provided by the laser 6.

In that the laser pulses or pulse trains are emitted in aposition-controlled manner, complex programming of the severing processis omitted. Freely selectable process speeds can moreover be implementedeasily.

FIG. 9 shows how a quasi-non-diffracting beam is introduced into thematerial 1 from the partial laser beams after a beamforming optical unit9. In FIG. 9A, the partial laser beams are incident on the surface 14 ofthe material symmetrically to the surface normal 140 of the material 1.In particular, the laser beam is thus incident as a whole at a rightangle on the surface 14. Correspondingly, the elongated modificationzone 602 is aligned in parallel to the surface normal 140, thus inparticular does not experience refraction. However, the partial laserbeams are very probably incident at an angle on the material surface 14,so that they are refracted according to Snell's law of refraction. Thelength of the elongated modification zone 602 in the material 1 may bedetermined by the index of refraction of the material 1 and the angle ofincidence of the partial laser beams. Material modifications 3 can beintroduced into the material 1 along the elongated modification zone602.

FIG. 9B shows a situation in which the partial laser beams are notintroduced into the material 1 symmetrically to the surface normal 140,but at an angle θ. An elongated modification zone 602 is thus formed inthe material, which does not extend in parallel to the surface normal140, but is refracted at a certain angle θ′. It is thus possible tointroduce material modifications 3 into the material 1 which do notextend in parallel to the surface normal 140. A material 1 can thus besevered at an angle θ′, for example.

FIG. 10 shows microscope pictures of the material modifications 3 whichhave been introduced into the material 1 for various pulse energies. Forthis purpose, the elongated modification zone 602 penetrated the surface14 of the material 1. Accordingly, the material modifications 3 showneach begin at the surface 14. At a pulse energy of 700 μJ, a firstelongated modification zone 602 was generated which was shorter than thematerial thickness L_(M). Accordingly, the material modification endsbefore it reaches the lower surface. To enlarge the elongatedmodification zones 602, the pulse energy was increased, as shown abovein particular in FIG. 3F. For example, at a pulse energy of 1400 μJ, anelongated modification zone 602 was generated which was twice as long asat 700 μJ. In principle, however, a linear relationship does not have toexist between the length of the elongated modification zone and thepulse energy. However, it is possible that the relationship betweenlength of the elongated modification zone and pulse energy can beapproximated in sections by a linear relationship. Accordingly, thegenerated elongated modification zone 602 was greater than 1.5×L_(M), sothat a material modification 3 was generated in the material 1 whichextends between the two opposing material surfaces.

If applicable, all individual features which are represented in theexemplary embodiments can be combined and/or exchanged with one anotherwithout leaving the area of the invention.

While subject matter of the present disclosure has been illustrated anddescribed in detail in the drawings and foregoing description, suchillustration and description are to be considered illustrative orexemplary and not restrictive. Any statement made herein characterizingthe invention is also to be considered illustrative or exemplary and notrestrictive as the invention is defined by the claims. It will beunderstood that changes and modifications may be made, by those ofordinary skill in the art, within the scope of the following claims,which may include any combination of features from different embodimentsdescribed above.

The terms used in the claims should be construed to have the broadestreasonable interpretation consistent with the foregoing description. Forexample, the use of the article “a” or “the” in introducing an elementshould not be interpreted as being exclusive of a plurality of elements.Likewise, the recitation of “or” should be interpreted as beinginclusive, such that the recitation of “A or B” is not exclusive of “Aand B,” unless it is clear from the context or the foregoing descriptionthat only one of A and B is intended. Further, the recitation of “atleast one of A, B and C” should be interpreted as one or more of a groupof elements consisting of A, B and C, and should not be interpreted asrequiring at least one of each of the listed elements A, B and C,regardless of whether A, B and C are related as categories or otherwise.Moreover, the recitation of “A, B and/or C” or “at least one of A, B orC” should be interpreted as including any singular entity from thelisted elements, e.g., A, any subset from the listed elements, e.g., Aand B, or the entire list of elements A, B and C.

LIST OF REFERENCE SIGNS

-   -   1 material    -   10 first material half    -   12 second material half    -   14 surface    -   140 surface normals    -   2 severing line    -   20 severing surface    -   3 material modification    -   30 material modification area    -   300 material ejection    -   32 crack    -   4 beam cross section    -   41 main order    -   43 secondary order    -   5 control device    -   6 laser    -   60 laser beam    -   600 focus zone    -   602 modification zone    -   62 continuous wave CO2 laser    -   620 temperature gradient    -   65 hollow core fibre    -   7 focusing unit    -   70 mirror    -   72 focusing optical units    -   8 feed device    -   80 first part of the feed device    -   800 support surface    -   82 second part of the feed device    -   9 beamforming optical unit    -   11 chemical bath    -   110 reaction bubbles    -   L_(M) thickness of the material    -   dA external diameter of the material ejection    -   dM spacing of the material modifications    -   FZ tensile force

1. A method for severing an at least partially transparent material, themethod comprising: focusing ultrashort laser pulses, as individual laserpulses and/or as pulse trains, in the material so that a resultingmodification zone elongated in a beam propagation direction enters thematerial and penetrates at least one surface of the material, whereineach pulse train comprises multiple sub-laser pulses, introducing aplurality of material modifications along a severing line into thematerial via the laser pulses, and severing the material along thesevering line, wherein a pulse energy of the individual laser pulses ora sum of pulse energies of the sub-laser pulses is in a range from 500μJ to 50 mJ, and a length of the modification zone in the beampropagation direction is greater than a thickness of the material L_(M).2. The method according to claim 1, wherein the length of themodification zone is greater than 1.5×L_(M).
 3. The method according toclaim 1, wherein the length of the modification zone is greater than2×(200 μm)+L_(M).
 4. The method according to claim 1, wherein severingthe material comprises applying a thermal stress along the severing lineand/or applying a mechanical stress, and/or performing etching using atleast one wet-chemical solution.
 5. The method according to claim 1,wherein the material comprises a glass substrate, and/or a stackedsubstrate system, and/or a silicon wafer.
 6. The method according toclaim 5, wherein the thickness of the material L_(M) is greater than 1mm.
 7. The method according to claim 1, wherein the laser pulses have awavelength between 0.3 μm and 1.5 μm and/or a pulse length of theindividual laser pulses and/or of the sub-laser pulses is in a rangefrom 0.01 ps to 50 ps, and/or an average power of a laser output isbetween 150 W and 15 kW.
 8. The method according to claim 1, wherein alaser beam formed by the laser pulses and the material are displaceablerelative to one another with a feed in order to introduce the pluralityof the material modifications into the material along the severing line,wherein the laser beam and the material are alignable in relation to oneanother at an angle via tilting and/or rotation.
 9. The method accordingto claim 1, wherein a maximum diameter of a beam cross sectionperpendicular to the beam propagation direction in the modification zoneis between 1 and 50 μm.
 10. The method according to claim 1, wherein alaser beam formed by the laser pulses comprises a quasi-non-diffractingbeam at least in the elongated modification zone.
 11. The methodaccording to claim 10, wherein the laser beam has a non-radiallysymmetric beam cross section perpendicular to the beam propagationdirection, wherein the beam cross section or an envelope of the beamcross section has an elliptical shape.
 12. The method according to claim11, wherein a long axis of the non-radially symmetric beam cross sectionis oriented perpendicular to the beam propagation direction along thesevering line and/or along the feed direction.
 13. The method accordingto claim 12, wherein the elliptical quasi-non-diffracting beam has anon-negligible interference contrast of less than 0.9 along the longaxis.
 14. The method according to claim 1, wherein a laser beam formedby the laser pulses is incident at a processing angle on the at leastone surface of the material, wherein the processing angle is not a rightangle.
 15. The method according to claim 14, wherein the processingangle is less than 20° for the thickness of the material being less than2 mm.
 16. The method according to claim 14, wherein the processing angleis less than 10° for the thickness of the material being greater than 2mm.
 17. The method according to claim 1, wherein the individual laserpulses and/or the pulse trains are triggered by a position-controlledpulse triggering from a laser system, wherein the position-controlledpulse triggering is based on a position of a laser beam formed by thelaser pulses on the material.